For some time, astronomers have known that collisions or mergers between galaxies are an integral part of cosmic evolution. In addition to causing galaxies to grow, these mergers also trigger new rounds of star formation as fresh gas and dust are injected into the galaxy. In the future, astronomers estimate that the Milky Way Galaxy will merge with the Andromeda Galaxy, as well as the Small and Large Magellanic Clouds in the meantime.
According to new results obtained by researchers at the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York city, the results of our eventual merger with the Magellanic Clouds is already being felt. According to results presented at the 235th meeting of the American Astronomical Society this week, stars forming in the outskirts of our galaxy could be the result of these dwarf galaxies merging with our own.
In the course of the presentation, which took place on Wed (Jan. 8th) in Honolulu, the research team explained how data from the ESA’s Gaia observatory revealed the existence of a young stellar cluster in the outskirts of the Milky Way’s halo. This cluster has been designated Price-Whelan 1 in honor of the team leader Adrian M. Price-Whelan (a research fellow with the CCA).
Even more surprising was the fact that spectra obtained from the cluster indicated that they likely formed from the stream of gas emanating from one of the arms of the Large Magellanic Cloud. The discovery suggests that this stream of gas extending from the galaxies, known as Leading Arm II, is considerably closer to the Milky Way than previously thought (and also closer to colliding with it).
To be sure, identifying star clusters in our galaxy is difficult since stars may appear to be clustered in the sky, but are separated by vast distances in reality. In addition, stars may be seen in proximity to each other at one point but then find themselves moving in different directions. Determining which stars are clustered together requires precise measurements of the positions stars over time (aka. astrometry).
This is the purpose of the Gaia mission, which has been gathering data on the positions, distances, and proper motions of about 1.7 billion celestial objects since 2013. Using the latest dataset to be released by the mission, Price-Whelan and his colleagues searched for evidence of very blue young stars that had clumps moving with them. After identifying several, they crossed-matched them to eliminate known clusters.
In the end, only one remained: a relatively young star cluster that is about 117 million years old and located on the far outskirts of the Milky Way. As Price-Whelan explained:
“This is a puny cluster of stars – less than a few thousand in total – but it has big implications beyond its local area of the Milky Way… It’s really, really far away. It’s further than any known young stars in the Milky Way, which are typically in the disk. So right away, I was like, ‘Holy smokes, what is this?'”
The cluster’s position places it in the Milky Way’s “halo”, the outer region of our galaxy located beyond the spiral arms. While it contains the majority of our galaxy’s mass, it is also much darker than the spiral arms where the majority of the Milky Way’s stars are located. Also located in this region is a river of gas known as the “Magellanic Stream”, which forms the outermost edge of the SMC and LMC and reaches toward the Milky Way.
This stream is metal-poor, unlike clouds of gas found in the outer reaches of the Milky Way. This allowed David Nidever, an assistant professor at Montana State University and a co-author of the study, to determine that the newfound star cluster was extragalactic in origin. By conducting an analysis of the metal content of the 27 brightest stars in the cluster, he found that their metallicity was similar to that of the Magellanic Stream.
Based on these findings, the team concluded that the cluster formed as gas from the Magellanic Stream passed through the Milky Way’s halo. Combined with the gravitational pull of our galaxy, passing through the halo created a drag force that compressed the gas to the point that it collapsed to form new stars. Over time, the stars moved ahead of the gas stream and joined the outer Milky Way.
The study of this cluster could have considerable implications for our understanding of our galaxy’s evolution. For example, astronomers have been unable to effectively constrain the distance between the Magellanic Stream and our galaxy until now. But thanks to the discovery of this new star cluster, Price-Whelan and his colleagues predict that the edge of the Magellanic Stream is 90,000 light-years away from the Milky Way.
That’s roughly half the distance that was previously predicted. In addition, the discovery of clusters in the outskirts of the Milky Way could also reveal whether the Magellanic Clouds collided with our galaxy in the past. This is the apparent tendency where mergers are concerned: the two celestial objects do not collide head-on, but swing past each other and exchange material, eventually coalescing to form a single object.
As Nidever indicated, the team’s findings are also leading astronomers to refine their theories about when the Large Magellanic Cloud will merge with our galaxy:
“If the Magellanic Stream is closer, especially the leading arm closest to our galaxy, then it’s likely to be incorporated into the Milky Way sooner than the current model predicts. Eventually, that gas will turn into new stars in the Milky Way’s disk. Right now, our galaxy is using up gas faster than its being replenished. This extra gas coming in will help us replenish that reservoir and make sure that our galaxy continues to thrive and form new stars.”
This study is the latest in a series that were made possible by the Gaia mission, which are collectively advancing our understanding of how our galaxy evolved and will continue to do so in the future. Initially planned to end by 2018, the Gaia mission has been extended and will remain in operation until 2022 (barring further extensions).
The next release of Gaia archive data (EDR3) will take place in two parts, with the first being released in the third quarter of 2020 and the second during the latter half of 2021. The discovery of Price-Whelan 1 and the team’s subsequent spectroscopic analysis of the stars were both the subject of papers that were published in The Astrophysical Journal on Dec. 5th and Dec. 16th, respectively.
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